1. Field of the Invention
This invention relates to bending of glass sheets, and in particular to a horizontal press bending arrangement wherein glass sheets are shaped between an upper female vacuum mold and a lower full surface male mold.
2a. Technical Considerations
Shaped and tempered glass sheets are widely used as side windows or rear windows in vehicles such as automobiles and the like. To be suitable for such applications, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings in the vehicle. It is also important that the side or rear windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat glass sheets to their heat softening temperature, shaping the heated glass to a desired curvature, and cooling the shaped sheets in a controlled manner to a temperature below the strain point temperature of the glass e.g., tempering, to strengthen the glass and increase its resistance to damage resulting from impact.
During such an operation, a glass sheet is conveyed along a substantially horizontal path that extends through a tunnel type furnace. The glass sheet is one of a series of sheets and is heated to its heat softening temperature and transferred, for example, by a vacuum pick-up and shuttle arrangement, into a shaping station adjacent to the furnace where the glass sheet is pressed between a pair of vertically aligned upper and lower shaping molds. The upper mold is generally a vacuum male mold, i.e., it has a generally convex sheet shaping surface that holds the heat softened glass sheet by suction thereagainst after shaping and the lower mold is generally a female mold, i.e., it has a generally concave sheet shaping surface complementing the shaping surface of the upper mold. After shaping, a transfer and tempering ring having an outline and elevational contour conforming to that desired for the glass sheet slightly inboard of its perimeter moves downstream into a position below the upper vacuum mold. Release of the vacuum deposits the glass sheet onto the tempering ring which supports the peripheral portion of the glass while it conveys the glass sheet into a cooling station for rapid cooling.
As stylists continue to strive toward more aerodynamic vehicle designs, the bent shape of the glass sheets are becoming more complex. These designs include small radius bends, reverse curvatures in both the longitudinal and transverse direction, and tight dimensional and contour tolerances. These types of shapes are becoming increasingly more difficult to form using conventional pressing techniques. The shaped glass sheets are generally formed by pressing the end portions of the heat softened sheets upward relative to the central portions of the sheet. As a result, in order to conform with the shaping surfaces of the molds, portions of the glass sheets are forced in a direction opposite to its natural tendency to sag downwardly due to gravity. In some instances, the glass sheets must be heated to temperatures well above the normal heat softening temperature of 1050.degree. F. to 1150.degree. F. (566.degree. C. to 621.degree. C.), so that the glass is pliable enough to attempt to form these complex shapes. In addition, it has been found that the shaped glass sheet will shrink in size from an initial configuration immediately after shaping to a final configuration after tempering. Depending on the overall configuration, this shrinkage may adversely affect the optical quality of the shaped glass sheet.
It would be advantageous to have a press bending arrangement that could form these complex shapes in the glass sheets while maintaining the quality optics required for use in vehicles.
2b. Patents of Interest
U.S. Pat. No. 4,290,796 to Reese et al teaches a glass sheet shaping apparatus for press bending glass doublets. Bending irons support the glass sheets as the irons are conveyed through a heating lehr, to preform the glass sheets by gravity sagging. The bending irons stop at a shaping station in approximate alignment between a pair of upper and lower vertically aligned pressing molds. The upper mold includes a full-face male pressing surface and the lower mold includes a full-face female pressing surface. The glass sheets are lifted off of the bending iron by the lower shaping mold and pressed between the complementing shaping surfaces. The shaped glass is then redeposited onto the bending iron which transfer the shaped glass out of the shaping station for subsequent cooling.
U.S. Pat. Nos. 4,221,580; 4,285,715; and 4,433,993 to Frank and U.S. Pat. No. 4,330,110 to Frank et al teach a horizontal press bending operation wherein heated glass sheets enter a shaping station and are lifted off run-in conveyor rolls by a slotted mold. The glass sheet is pressed between the slotted lower mold and a shaped upper vacuum mold having a full-face male pressing surface. After pressing, the lower mold is retracted to a position beneath the run in rolls. A shuttling tempering ring is positioned below the vacuum mold and the vacuum is released so that the shaped glass is deposited onto the tempering ring. The ring subsequently transfers the shaped glass to a quenching station to temper the bent glass sheet.
U.S. Pat. No. 4,297,118 to Kellar et al teaches a shuttling, deformable vacuum mold that engages a heated glass sheet within a heating furnace. The deformable mold may change its surface engaging configuration from a flat surface to a convex shaping surface while engaging the glass sheet. The deformable vacuum mold deposits the shaped glass sheet onto a shuttling tempering ring that is positioned beneath the mold. After depositing the glass, the vacuum mold shuttles to a position outside of the furnace to cool prior to reentering the furnace to engage the next glass sheet.
U.S. Pat. No. 4,508,556 to Bennett et al teaches a method of press bending a glass sheet to an S-shaped cross-sectional configuration. The glass sheet is conveyed on a gas hearth support where it is heated to its heat softening temperature and transferred into a shaping station which includes an auxiliary glass hearth to support the glass sheet. The downstream end of the auxiliary gas hearth is curved downward to impart an initial curved configuration to the heated glass sheet. A ring-type shaping mold which surrounds the gas hearth lifts the glass sheet and presses it against an upper full-face vacuum mold to impart the final shaped configuration. After shaping, the ring mold is retracted and a tempering ring is moved beneath the upper vacuum male mold to receive the shaped glass sheet and remove it for tempering.